The present invention relates to magnetic resonance (MR) imaging. More particularly it relates to an improved invasive device capable of providing position and orientation information in an MR system. The invention further relates to a method of rapidly acquiring information on both the position and orientation of an invasive device to dynamically define MR scan planes for continuous tracking of the invasive device. The invasive device tracking information and dynamic MR imaging are used to show both the invasive device and the target to enhance target-navigation.
In invasive MR guided procedures, reliable and accurate visualization of surgical and interventional instruments hereinafter referred to as invasive devices, inside the body of the subject is essential for procedure success. Micro radio frequency (RF) coils have been used for MR tracking of invasive devices. In typical applications, one small RF coil integrated into the tip of an invasive device detects RF signals from the immediate surroundings and the tip position is calculated from the detected MRI signals.
During previous device tracking procedures, graphical markers representing the device tip are overlain on pre-acquired, static roadmap images of the subject. Roadmap images are acceptable for invasive procedures performed on subjects having little motion. However the static roadmap may contribute to misregistration error due to subject movement that is likely to occur, for example, in abdominal invasive procedures. Moreover, when the invasive device trajectory is complex, such as in intravascular procedures, it is quite possible for the device to deviate from the roadmap scan plane. Accordingly the simple overlay of a graphical marker on a static roadmap image can lead to an incorrect representation of the true position of the device.
Device orientation and tip position together provide device trajectory information sufficient for accurate MRI guided interventional procedures. However significant challenges exist in acquiring the trajectory information. Multiple coils disposed on the device have been used in attempt to provide the locations of several points on the invasive device for determining the device's trajectory. However such approaches have not solved the problems inherent with standing wave generation from multiple leads connected to the coils.
Other attempts have used a single coil with multiple windings to provide the location information of several points on the invasive device. However such attempts have not provided the necessary unique correspondence between the winding elements and the 1DFT peaks since the signals from each winding element are induced simultaneously. For example, if a single coil has two winding elements, two peaks (x1 and x2, or y1 and y2, or z1 and z2) will be detected from a gradient echo along any one of three orthogonal axes (x, y, or z axis). There are two possible ways to assign these two peaks to the spatial coordinates of the two winding elements, e.g. (x1, x2) or (x2, x1). In total, there will be 2*2*2=8 possible combinations of (x1, y1, z1) and (x2, y2, z2) which can be assigned to the coordinates of the coils only two of which are the true coil locations.
A second problem, known as peak ambiguity, exists with using multiple windings. When the field gradient is applied almost orthogonal to the axis of the single multi-element micro coil, each coil element may lie at approximately the same coordinate along the gradient axis and hence induce signals at similar frequencies. The multiple peaks in the 1DFT may not fall beyond the spectral resolution of the acquisition so that the normally separate peaks merge into only one. Thus valuable information on one of the coordinates may not be available to the precision needed for guidance.
Accordingly, it has been considered desirable to develop a new and improved invasive device and method of guiding an invasive device using target navigation.